Rollup merge of #137449 - compiler-errors:control-flow, r=Amanieu,lnicola

Denote `ControlFlow` as `#[must_use]`

I've repeatedly hit bugs in the compiler due to `ControlFlow` not being marked `#[must_use]`. There seems to be an accepted ACP to make the type `#[must_use]` (https://github.com/rust-lang/libs-team/issues/444), so this PR implements that part of it.

Most of the usages in the compiler that trigger this new warning are "root" usages (calling into an API that uses control-flow internally, but for which the callee doesn't really care) and have been suppressed by `let _ = ...`, but I did legitimately find one instance of a missing `?` and one for a never-used `ControlFlow` value in #137448.

Presumably this needs an FCP too, so I'm opening this and nominating it for T-libs-api.

This PR also touches the tools (incl. rust-analyzer), but if this went into FCP, I'd split those out into separate PRs which can land before this one does.

r? libs-api
`@rustbot` label: T-libs-api I-libs-api-nominated

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -2136,7 +2136,7 @@ fn add_library_search_dirs(
     }
 
     let fallback = Some(NativeLibSearchFallback { self_contained_components, apple_sdk_root });
-    walk_native_lib_search_dirs(sess, fallback, |dir, is_framework| {
+    let _ = walk_native_lib_search_dirs(sess, fallback, |dir, is_framework| {
         if is_framework {
             cmd.framework_path(dir);
         } else {

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -533,7 +533,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         }
                     }
                     _ => {
-                        intravisit::walk_pat(self, p);
+                        let _ = intravisit::walk_pat(self, p);
                     }
                 }
                 ControlFlow::Continue(())
@@ -556,7 +556,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     method_name,
                     sugg_let: None,
                 };
-                let_visitor.visit_body(&body);
+                let _ = let_visitor.visit_body(&body);
                 if let Some(sugg_let) = let_visitor.sugg_let
                     && let Some(self_ty) = self.node_ty_opt(sugg_let.init_hir_id)
                 {

compiler/rustc_privacy/src/lib.rs

@@ -1361,12 +1361,12 @@ impl SearchInterfaceForPrivateItemsVisitor<'_> {
                 GenericParamDefKind::Lifetime => {}
                 GenericParamDefKind::Type { has_default, .. } => {
                     if has_default {
-                        self.visit(self.tcx.type_of(param.def_id).instantiate_identity());
+                        let _ = self.visit(self.tcx.type_of(param.def_id).instantiate_identity());
                     }
                 }
                 // FIXME(generic_const_exprs): May want to look inside const here
                 GenericParamDefKind::Const { .. } => {
-                    self.visit(self.tcx.type_of(param.def_id).instantiate_identity());
+                    let _ = self.visit(self.tcx.type_of(param.def_id).instantiate_identity());
                 }
             }
         }
@@ -1381,19 +1381,19 @@ impl SearchInterfaceForPrivateItemsVisitor<'_> {
         // consider the ones that the user wrote. This is important
         // for the inferred outlives rules; see
         // `tests/ui/rfc-2093-infer-outlives/privacy.rs`.
-        self.visit_predicates(self.tcx.explicit_predicates_of(self.item_def_id));
+        let _ = self.visit_predicates(self.tcx.explicit_predicates_of(self.item_def_id));
         self
     }
 
     fn bounds(&mut self) -> &mut Self {
         self.in_primary_interface = false;
-        self.visit_clauses(self.tcx.explicit_item_bounds(self.item_def_id).skip_binder());
+        let _ = self.visit_clauses(self.tcx.explicit_item_bounds(self.item_def_id).skip_binder());
         self
     }
 
     fn ty(&mut self) -> &mut Self {
         self.in_primary_interface = true;
-        self.visit(self.tcx.type_of(self.item_def_id).instantiate_identity());
+        let _ = self.visit(self.tcx.type_of(self.item_def_id).instantiate_identity());
         self
     }
 
@@ -1785,7 +1785,7 @@ fn check_mod_privacy(tcx: TyCtxt<'_>, module_def_id: LocalModDefId) {
 
     let module = tcx.hir_module_items(module_def_id);
     for def_id in module.definitions() {
-        rustc_ty_utils::sig_types::walk_types(tcx, def_id, &mut visitor);
+        let _ = rustc_ty_utils::sig_types::walk_types(tcx, def_id, &mut visitor);
 
         if let Some(body_id) = tcx.hir_maybe_body_owned_by(def_id) {
             visitor.visit_nested_body(body_id.id());
@@ -1798,7 +1798,11 @@ fn check_mod_privacy(tcx: TyCtxt<'_>, module_def_id: LocalModDefId) {
                 let trait_ref = tcx.impl_trait_ref(id.owner_id.def_id).unwrap();
                 let trait_ref = trait_ref.instantiate_identity();
                 visitor.span = item.path.span;
-                visitor.visit_def_id(trait_ref.def_id, "trait", &trait_ref.print_only_trait_path());
+                let _ = visitor.visit_def_id(
+                    trait_ref.def_id,
+                    "trait",
+                    &trait_ref.print_only_trait_path(),
+                );
             }
         }
     }

compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/find_anon_type.rs

@@ -77,7 +77,7 @@ impl<'tcx> Visitor<'tcx> for FindNestedTypeVisitor<'tcx> {
         match arg.kind {
             hir::TyKind::BareFn(_) => {
                 self.current_index.shift_in(1);
-                intravisit::walk_ty(self, arg);
+                let _ = intravisit::walk_ty(self, arg);
                 self.current_index.shift_out(1);
                 return ControlFlow::Continue(());
             }
@@ -85,7 +85,7 @@ impl<'tcx> Visitor<'tcx> for FindNestedTypeVisitor<'tcx> {
             hir::TyKind::TraitObject(bounds, ..) => {
                 for bound in bounds {
                     self.current_index.shift_in(1);
-                    self.visit_poly_trait_ref(bound);
+                    let _ = self.visit_poly_trait_ref(bound);
                     self.current_index.shift_out(1);
                 }
             }

compiler/rustc_trait_selection/src/traits/project.rs

@@ -743,7 +743,7 @@ fn assemble_candidates_from_trait_def<'cx, 'tcx>(
 ) {
     debug!("assemble_candidates_from_trait_def(..)");
     let mut ambiguous = false;
-    selcx.for_each_item_bound(
+    let _ = selcx.for_each_item_bound(
         obligation.predicate.self_ty(),
         |selcx, clause, _| {
             let Some(clause) = clause.as_projection_clause() else {

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -176,7 +176,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             // normalization, so try to deduplicate when possible to avoid
             // unnecessary ambiguity.
             let mut distinct_normalized_bounds = FxHashSet::default();
-            self.for_each_item_bound::<!>(
+            let _ = self.for_each_item_bound::<!>(
                 placeholder_trait_predicate.self_ty(),
                 |selcx, bound, idx| {
                     let Some(bound) = bound.as_trait_clause() else {

compiler/rustc_type_ir/src/binder.rs

@@ -112,7 +112,7 @@ where
     pub fn bind_with_vars(value: T, bound_vars: I::BoundVarKinds) -> Binder<I, T> {
         if cfg!(debug_assertions) {
             let mut validator = ValidateBoundVars::new(bound_vars);
-            value.visit_with(&mut validator);
+            let _ = value.visit_with(&mut validator);
         }
         Binder { value, bound_vars }
     }
@@ -196,7 +196,7 @@ impl<I: Interner, T> Binder<I, T> {
         let value = f(value);
         if cfg!(debug_assertions) {
             let mut validator = ValidateBoundVars::new(bound_vars);
-            value.visit_with(&mut validator);
+            let _ = value.visit_with(&mut validator);
         }
         Binder { value, bound_vars }
     }
@@ -209,7 +209,7 @@ impl<I: Interner, T> Binder<I, T> {
         let value = f(value)?;
         if cfg!(debug_assertions) {
             let mut validator = ValidateBoundVars::new(bound_vars);
-            value.visit_with(&mut validator);
+            let _ = value.visit_with(&mut validator);
         }
         Ok(Binder { value, bound_vars })
     }

library/core/src/ops/control_flow.rs

@@ -80,6 +80,7 @@ use crate::{convert, ops};
 /// [`Continue`]: ControlFlow::Continue
 #[stable(feature = "control_flow_enum_type", since = "1.55.0")]
 #[rustc_diagnostic_item = "ControlFlow"]
+#[must_use]
 // ControlFlow should not implement PartialOrd or Ord, per RFC 3058:
 // https://rust-lang.github.io/rfcs/3058-try-trait-v2.html#traits-for-controlflow
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]

src/librustdoc/html/length_limit/tests.rs

@@ -9,41 +9,41 @@ fn empty() {
 #[test]
 fn basic() {
     let mut buf = HtmlWithLimit::new(60);
-    buf.push("Hello ");
+    let _ = buf.push("Hello ");
     buf.open_tag("em");
-    buf.push("world");
+    let _ = buf.push("world");
     buf.close_tag();
-    buf.push("!");
+    let _ = buf.push("!");
     assert_eq!(buf.finish(), "Hello <em>world</em>!");
 }
 
 #[test]
 fn no_tags() {
     let mut buf = HtmlWithLimit::new(60);
-    buf.push("Hello");
+    let _ = buf.push("Hello");
-    buf.push(" world!");
+    let _ = buf.push(" world!");
     assert_eq!(buf.finish(), "Hello world!");
 }
 
 #[test]
 fn limit_0() {
     let mut buf = HtmlWithLimit::new(0);
-    buf.push("Hello ");
+    let _ = buf.push("Hello ");
     buf.open_tag("em");
-    buf.push("world");
+    let _ = buf.push("world");
     buf.close_tag();
-    buf.push("!");
+    let _ = buf.push("!");
     assert_eq!(buf.finish(), "");
 }
 
 #[test]
 fn exactly_limit() {
     let mut buf = HtmlWithLimit::new(12);
-    buf.push("Hello ");
+    let _ = buf.push("Hello ");
     buf.open_tag("em");
-    buf.push("world");
+    let _ = buf.push("world");
     buf.close_tag();
-    buf.push("!");
+    let _ = buf.push("!");
     assert_eq!(buf.finish(), "Hello <em>world</em>!");
 }
 
@@ -51,11 +51,11 @@ fn exactly_limit() {
 fn multiple_nested_tags() {
     let mut buf = HtmlWithLimit::new(60);
     buf.open_tag("p");
-    buf.push("This is a ");
+    let _ = buf.push("This is a ");
     buf.open_tag("em");
-    buf.push("paragraph");
+    let _ = buf.push("paragraph");
     buf.open_tag("strong");
-    buf.push("!");
+    let _ = buf.push("!");
     buf.close_tag();
     buf.close_tag();
     buf.close_tag();
@@ -66,11 +66,11 @@ fn multiple_nested_tags() {
 fn forgot_to_close_tags() {
     let mut buf = HtmlWithLimit::new(60);
     buf.open_tag("p");
-    buf.push("This is a ");
+    let _ = buf.push("This is a ");
     buf.open_tag("em");
-    buf.push("paragraph");
+    let _ = buf.push("paragraph");
     buf.open_tag("strong");
-    buf.push("!");
+    let _ = buf.push("!");
     assert_eq!(buf.finish(), "<p>This is a <em>paragraph<strong>!</strong></em></p>");
 }
 
@@ -78,10 +78,10 @@ fn forgot_to_close_tags() {
 fn past_the_limit() {
     let mut buf = HtmlWithLimit::new(20);
     buf.open_tag("p");
-    (0..10).try_for_each(|n| {
+    let _ = (0..10).try_for_each(|n| {
         buf.open_tag("strong");
-        buf.push("word#")?;
+        let _ = buf.push("word#")?;
-        buf.push(&n.to_string())?;
+        let _ = buf.push(&n.to_string())?;
         buf.close_tag();
         ControlFlow::Continue(())
     });
@@ -100,8 +100,8 @@ fn past_the_limit() {
 fn quickly_past_the_limit() {
     let mut buf = HtmlWithLimit::new(6);
     buf.open_tag("p");
-    buf.push("Hello");
+    let _ = buf.push("Hello");
-    buf.push(" World");
+    let _ = buf.push(" World");
     // intentionally not closing <p> before finishing
     assert_eq!(buf.finish(), "<p>Hello</p>");
 }
@@ -110,7 +110,7 @@ fn quickly_past_the_limit() {
 fn close_too_many() {
     let mut buf = HtmlWithLimit::new(60);
     buf.open_tag("p");
-    buf.push("Hello");
+    let _ = buf.push("Hello");
     buf.close_tag();
     // This call does not panic because there are valid cases
     // where `close_tag()` is called with no tags left to close.

src/librustdoc/html/markdown.rs

@@ -1568,7 +1568,7 @@ fn markdown_summary_with_limit(
 
     let mut buf = HtmlWithLimit::new(length_limit);
     let mut stopped_early = false;
-    p.try_for_each(|event| {
+    let _ = p.try_for_each(|event| {
         match &event {
             Event::Text(text) => {
                 let r =

src/tools/clippy/clippy_lints/src/loops/mut_range_bound.rs

@@ -129,7 +129,7 @@ impl BreakAfterExprVisitor {
         };
 
         get_enclosing_block(cx, hir_id).is_some_and(|block| {
-            visitor.visit_block(block);
+            let _ = visitor.visit_block(block);
             visitor.break_after_expr
         })
     }

src/tools/clippy/clippy_lints/src/methods/read_line_without_trim.rs

@@ -40,7 +40,7 @@ pub fn check(cx: &LateContext<'_>, call: &Expr<'_>, recv: &Expr<'_>, arg: &Expr<
         // We've checked that `call` is a call to `Stdin::read_line()` with the right receiver,
         // now let's check if the first use of the string passed to `::read_line()`
         // is used for operations that will always fail (e.g. parsing "6\n" into a number)
-        for_each_local_use_after_expr(cx, local_id, call.hir_id, |expr| {
+        let _ = for_each_local_use_after_expr(cx, local_id, call.hir_id, |expr| {
             if let Some(parent) = get_parent_expr(cx, expr) {
                 let data = if let ExprKind::MethodCall(segment, recv, args, span) = parent.kind {
                     if args.is_empty()

src/tools/clippy/clippy_lints/src/pass_by_ref_or_value.rs

@@ -141,7 +141,7 @@ impl PassByRefOrValue {
         // Gather all the lifetimes found in the output type which may affect whether
         // `TRIVIALLY_COPY_PASS_BY_REF` should be linted.
         let mut output_regions = FxHashSet::default();
-        for_each_top_level_late_bound_region(fn_sig.skip_binder().output(), |region| -> ControlFlow<!> {
+        let _ = for_each_top_level_late_bound_region(fn_sig.skip_binder().output(), |region| -> ControlFlow<!> {
             output_regions.insert(region);
             ControlFlow::Continue(())
         });

src/tools/clippy/clippy_lints/src/unconditional_recursion.rs

@@ -381,7 +381,7 @@ impl UnconditionalRecursion {
                 implemented_ty_id,
                 method_span,
             };
-            walk_body(&mut c, body);
+            let _ = walk_body(&mut c, body);
         }
     }
 }

src/tools/rust-analyzer/crates/hir-ty/src/chalk_db.rs

@@ -145,19 +145,19 @@ impl chalk_solve::RustIrDatabase<Interner> for ChalkContext<'_> {
         let mut result = vec![];
         if fps.is_empty() {
             debug!("Unrestricted search for {:?} impls...", trait_);
-            self.for_trait_impls(trait_, self_ty_fp, |impls| {
+            let _ = self.for_trait_impls(trait_, self_ty_fp, |impls| {
                 result.extend(impls.for_trait(trait_).map(id_to_chalk));
                 ControlFlow::Continue(())
-            })
+            });
         } else {
-            self.for_trait_impls(trait_, self_ty_fp, |impls| {
+            let _ = self.for_trait_impls(trait_, self_ty_fp, |impls| {
                 result.extend(
                     fps.iter().flat_map(move |fp| {
                         impls.for_trait_and_self_ty(trait_, *fp).map(id_to_chalk)
                     }),
                 );
                 ControlFlow::Continue(())
-            })
+            });
         };
 
         debug!("impls_for_trait returned {} impls", result.len());

src/tools/rust-analyzer/crates/hir-ty/src/dyn_compatibility.rs

@@ -116,7 +116,7 @@ pub fn dyn_compatibility_of_trait_query(
     trait_: TraitId,
 ) -> Option<DynCompatibilityViolation> {
     let mut res = None;
-    dyn_compatibility_of_trait_with_callback(db, trait_, &mut |osv| {
+    let _ = dyn_compatibility_of_trait_with_callback(db, trait_, &mut |osv| {
         res = Some(osv);
         ControlFlow::Break(())
     });
@@ -597,7 +597,7 @@ fn contains_illegal_impl_trait_in_trait(
 
     let ret = sig.skip_binders().ret();
     let mut visitor = OpaqueTypeCollector(FxHashSet::default());
-    ret.visit_with(visitor.as_dyn(), DebruijnIndex::INNERMOST);
+    let _ = ret.visit_with(visitor.as_dyn(), DebruijnIndex::INNERMOST);
 
     // Since we haven't implemented RPITIT in proper way like rustc yet,
     // just check whether `ret` contains RPIT for now

src/tools/rust-analyzer/crates/hir-ty/src/dyn_compatibility/tests.rs

@@ -53,7 +53,7 @@ fn check_dyn_compatibility<'a>(
             continue;
         };
         let mut osvs = FxHashSet::default();
-        dyn_compatibility_with_callback(&db, trait_id, &mut |osv| {
+        let _ = dyn_compatibility_with_callback(&db, trait_id, &mut |osv| {
             osvs.insert(match osv {
                 DynCompatibilityViolation::SizedSelf => SizedSelf,
                 DynCompatibilityViolation::SelfReferential => SelfReferential,

src/tools/rust-analyzer/crates/hir-ty/src/infer.rs

@@ -1143,7 +1143,7 @@ impl<'a> InferenceContext<'a> {
             non_assocs: FxHashMap::default(),
         };
         for ty in tait_candidates {
-            ty.visit_with(collector.as_dyn(), DebruijnIndex::INNERMOST);
+            let _ = ty.visit_with(collector.as_dyn(), DebruijnIndex::INNERMOST);
         }
 
         // Non-assoc TAITs can be define-used everywhere as long as they are

src/tools/rust-analyzer/crates/hir-ty/src/lib.rs

@@ -1033,7 +1033,7 @@ where
     T: ?Sized + TypeVisitable<Interner>,
 {
     let mut collector = PlaceholderCollector { db, placeholders: FxHashSet::default() };
-    value.visit_with(&mut collector, DebruijnIndex::INNERMOST);
+    let _ = value.visit_with(&mut collector, DebruijnIndex::INNERMOST);
     collector.placeholders.into_iter().collect()
 }
 

src/tools/rust-analyzer/crates/hir-ty/src/method_resolution.rs

@@ -596,7 +596,7 @@ pub(crate) fn iterate_method_candidates<T>(
     mut callback: impl FnMut(ReceiverAdjustments, AssocItemId, bool) -> Option<T>,
 ) -> Option<T> {
     let mut slot = None;
-    iterate_method_candidates_dyn(
+    let _ = iterate_method_candidates_dyn(
         ty,
         db,
         env,

src/tools/rust-analyzer/crates/hir/src/attrs.rs

@@ -260,7 +260,7 @@ fn resolve_impl_trait_item(
     // attributes here. Use path resolution directly instead.
     //
     // FIXME: resolve type aliases (which are not yielded by iterate_path_candidates)
-    method_resolution::iterate_path_candidates(
+    let _ = method_resolution::iterate_path_candidates(
         &canonical,
         db,
         environment,

src/tools/rust-analyzer/crates/hir/src/lib.rs

@@ -2911,7 +2911,7 @@ impl Trait {
         db: &dyn HirDatabase,
     ) -> Option<Vec<DynCompatibilityViolation>> {
         let mut violations = vec![];
-        hir_ty::dyn_compatibility::dyn_compatibility_with_callback(db, self.id, &mut |violation| {
+        let _ = hir_ty::dyn_compatibility::dyn_compatibility_with_callback(db, self.id, &mut |violation| {
             violations.push(violation);
             ControlFlow::Continue(())
         });
@@ -5497,7 +5497,7 @@ impl Type {
             .generic_def()
             .map_or_else(|| TraitEnvironment::empty(krate.id), |d| db.trait_environment(d));
 
-        method_resolution::iterate_method_candidates_dyn(
+        let _ = method_resolution::iterate_method_candidates_dyn(
             &canonical,
             db,
             environment,
@@ -5584,7 +5584,7 @@ impl Type {
             .generic_def()
             .map_or_else(|| TraitEnvironment::empty(krate.id), |d| db.trait_environment(d));
 
-        method_resolution::iterate_path_candidates(
+        let _ = method_resolution::iterate_path_candidates(
             &canonical,
             db,
             environment,

src/tools/rust-analyzer/crates/ide-assists/src/handlers/extract_function.rs

@@ -750,7 +750,7 @@ impl FunctionBody {
                         ast::Stmt::Item(_) => (),
                         ast::Stmt::LetStmt(stmt) => {
                             if let Some(pat) = stmt.pat() {
-                                walk_pat(&pat, &mut |pat| {
+                                let _ = walk_pat(&pat, &mut |pat| {
                                     cb(pat);
                                     std::ops::ControlFlow::<(), ()>::Continue(())
                                 });

src/tools/rust-analyzer/crates/ide-db/src/syntax_helpers/node_ext.rs

@@ -121,7 +121,7 @@ pub fn walk_patterns_in_expr(start: &ast::Expr, cb: &mut dyn FnMut(ast::Pat)) {
         match ast::Stmt::cast(node.clone()) {
             Some(ast::Stmt::LetStmt(l)) => {
                 if let Some(pat) = l.pat() {
-                    walk_pat(&pat, &mut |pat| {
+                    let _ = walk_pat(&pat, &mut |pat| {
                         cb(pat);
                         ControlFlow::<(), ()>::Continue(())
                     });
@@ -159,7 +159,7 @@ pub fn walk_patterns_in_expr(start: &ast::Expr, cb: &mut dyn FnMut(ast::Pat)) {
                     }
                 } else if let Some(pat) = ast::Pat::cast(node) {
                     preorder.skip_subtree();
-                    walk_pat(&pat, &mut |pat| {
+                    let _ = walk_pat(&pat, &mut |pat| {
                         cb(pat);
                         ControlFlow::<(), ()>::Continue(())
                     });